A permanganate-rare earth metal salt (REMS) chemical conversion bath was applied to a sample of AZ91 magnesium alloy in this study, a red-brown conversion coating formed subsequently on the sample surface. The test results of this coating with a scanning electron microscope (SEM) showed that there existed net-like cracks on the surface of the treated magnesium alloy. With the analyses of Xray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS), a further study of this coating indicated that the coating was structurally amorphous and mainly composed of CeO 2 , MnO, MnO 2 , MgO, Mg(OH) 2 and MgAl 2 O 4 . Furthermore, the electrochemical polarization tests showed that compared with the sam-ples treated by the chrome-based method, the open-circuit potential of the magnesium alloy coated in permanganate-REMS bath moved from Γ 1.34 V SCE to Γ 1.28 V SCE and the anodic current density of the alloy, at the same potential, decreased evidently in simulated sweat fluid. The cracks in the chemical conversion coating should be caused by the phase structure of the magnesium alloy. During the chemical conversion process, the localized corrosion micro-cell led to the formation of the net-like cracks on the surface. Simultaneously, the dehydration of the surface coating after treatment also accelerated the formation of the cracks at the coating surface.